1. Field of the Invention
The invention relates to a method for testing the adhesive strength of overlays attached to the inside of plain bearing bushings. The invention also relates to an apparatus for testing said adhesive strength.
2. Related Art
Plain bearing bushings generally comprise a steel backing and an overlay, which may consist of a plastics material or a metal alloy. A sintered layer, which may consist of sintered bronze for example, is often arranged between the overlay and the steel backing. It is such plain bearing bushings which the method according to the invention is designed to test.
Plain bearing bushings serve, as a rule, for mounting rotating shafts, wherein the lubricant is moved in the circumferential direction in the lubricant gap. Hydrodynamic lubrication is thus produced.
However, there are also applications in which the lubricant flows at the same time in the axial direction, which may lead to flow erosion in the overlay and optionally in the layers therebelow. This flow erosion may lead to detachment of the overlay and thus to failure of the bearing bushing.
Such flow conditions prevail, for example, in diesel injection pumps, in which the diesel fuel assumes the function of the lubricant. The bushings serve therein in particularly for mounting eccentric shafts, with the consequence that the bearing bushings are periodically also loaded in a radial direction, such that high axial flow velocities may arise in the then widening lubrication gap. The higher the flow velocity, the greater the risk of flow erosion.
As a result of the flow, the overlay is exposed to high shear stress. It is therefore desirable to simulate this shear stress under different conditions and to test the adhesive strength of the overlay.
The only test methods and apparatuses which are known are those with which test the tensile strength of layers of a flat composite material.
A test method is known from EP 0 212 694 which determines the adhesion of an overlay to a bronze layer of a flat multilayer plain bearing material using a testing apparatus according to ISO 4624. In this method the roughened flat end face of a testing bar of the testing apparatus is adhered to the overlay.
This method is complex and does not provide any information 25 about the resistance of the overlay to shear stress, which is caused inter alia by flow of a liquid medium.
Depending on the field of use, the plain bearing bushing is also subjected to axial load, in this case generally abruptly or in pulsed manner, wherein the shaft extending through the bushing in each case undergoes bending, leading on the one hand to edge pressure, loading of the bushing edge, and on the other hand to abrupt inflow of the liquid medium into the gap opening up between shaft and bushing.
DE-PS 22 61 789 discloses a machine component with a 5 coating for preventing chemical wear. A bearing testing machine is described for testing purposes which is equipped with a test shaft, an interchangeable steel bushing being attached thereto with sliding fit. Two connecting rod bearings with connecting rod are positioned on the steel or test bushing. To prevent the test bushing from rotating relative to the test shaft, the bushing is additionally secured by a tongue-and-groove arrangement. Interchangeable unbalanced flywheels may be used to subject the tight-fitting test bushing to a larger or smaller degree of vibration by means of the rotating test shaft. This patent specification does not provide any information about testing the adhesive strength of overlays attached to the inside of plain bearing bushings using purposeful application and inflow of a liquid medium.
DE 28 40 425 C3 also merely describes a plain bearing test rig in which, for the tests, in each case individual plain bearing halves are mounted in the lower part of the bearing receptacle. To establish test conditions, static load is applied just to the upper, untreated plain bearing half. A relatively large bearing clearance in the plain bearing half to be tested is additionally used to ensure that the journal does not come into mechanical contact with the test shell on start-up. Once the test conditions have been set, the plain bearing half to be tested is subjected to static load by means of a continuously adjustable hydraulic cylinder by changing the loading direction by means of a lever arm on the bearing receptacle.
It is therefore an object of the invention to provide a method and an apparatus with which the adhesive strength of overlays of plain bearing bushings under shock load may be tested simply.